Collective Depinning Research Articles

Triple-top-gate technique for studying the strongly interacting 2D electron systems in heterostructures

We have developed a technique that dramatically reduces the contact resistances and depletes a shunting channel between the contacts outside the Hall bar in ultra-high mobility SiGe/Si/SiGe heterostructures. It involves the creation of three overlapping independent gates deposited on top of the structure and allows transport measurements to be performed at millikelvin temperatures in the strongly interacting limit at low electron densities, where the energy of the electron–electron interactions dominates all other energy scales. This design allows one to observe the two-threshold voltage–current characteristics that are a signature for the collective depinning and sliding of the electron solid.

Collective depinning and sliding of a quantum Wigner solid in a two-dimensional electron system

Collective depinning and sliding of a quantum Wigner solid in a two-dimensional electron system

Collective depinning of driven monolayer active colloidal particles with magnetic dipole and Mie-type interactions

We report Langevin simulations on the collective depinning of driven monolayer active colloidal particles over the disordered substrate. The active colloidal particles are modeled as Janus particles interacting with magnetic dipole and Mie-type potentials. On increasing the Mie-type potential exponent , we find a peak of critical pinning force at . Above the depinning, there are living islands when the exponent is less than , and they disappear once is larger than . Further increasing the exponent will lead to moving crystal structures above the depinning. Moreover, the peak of critical pinning force occurs at a weaker strength of magnetic-dipole interaction as is increased to be a larger value, and it disappears as is further increased over . Finally, the critical pinning force remains basically unchanged with the strength of Mie-type potential, except for the system of , where the critical pinning force decreases with the strength of Mie-type potential. Our results are helpful to understand the formation of self-organisation in electromagnetic interaction biological systems driven by external field.

Transport evidence for a sliding two-dimensional quantum electron solid

Ignited by the discovery of the metal-insulator transition, the behaviour of low-disorder two-dimensional (2D) electron systems is currently the focus of a great deal of attention. In the strongly interacting limit, electrons are expected to crystallize into a quantum Wigner crystal, but no definitive evidence for this effect has been obtained despite much experimental effort over the years. Here, studying the insulating state of a 2D electron system in silicon, we have found two-threshold voltage-current characteristics with a dramatic increase in noise between the two threshold voltages. This behaviour cannot be described within existing traditional models. On the other hand, it is strikingly similar to that observed for the collective depinning of the vortex lattice in type-II superconductors. Adapting the model used for vortexes to the case of an electron solid yields good agreement with our experimental results, favouring the quantum electron solid as the origin of the low-density state.

Negative Longitudinal Magnetoresistance in the Density Wave Phase of Y2Ir2O7

The ground state of nanowires of single-crystalline pyrochlore Y_{2}Ir_{2}O_{7} is a density wave. The application of a transverse magnetic field increases the threshold electric field for the collective depinning of the density wave state at a low temperature, leading to colossal magnetoresistance for voltages around the depinning threshold. This is in striking contrast to the case where even a vanishingly small longitudinal magnetic field sharply reduces the depinning threshold voltage, resulting in negative magnetoresistance. Ruling out several other possibilities, we argue that this phenomenon is likely to be a consequence of the chiral anomaly in the gapped out Weyl semimetal phase in Y_{2}Ir_{2}O_{7}.

Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow.

Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model that maps the dynamics of the shear-driven colloidal layers to the motion of a single particle driven over an effective substrate potential. This model allows us to estimate the critical shear rate of the depinning transition based on the equilibrium configuration, revealing the impact of important parameters, such as the slit-pore width and the interaction strength. We then turn to heterogeneous systems where a layer of small colloids is sheared with respect to bottom layers of large particles. For these incommensurate systems we find that the particle transport is dominated by density excitations resembling the so-called "kink" solutions of the Frenkel-Kontorova (FK) model. In contrast to the FK model, however, the corresponding "antikinks" do not move.

Electric-field breakdown of the insulating charge-ordered state in LuFe2O4 thin films

We have studied the dielectric breakdown of the insulating charge-ordered state and the associated current switching phenomenon in layered ferrite LuFe2O4. To elucidate the correlation between the ordering pattern and current switching behaviour, we synthesized highly c-axis oriented thin films by pulsed-laser deposition. An enhanced switching effect was achieved in the three-dimensional charge-ordered phase below ∼310 K, but not in the high-temperature two-dimensional phase. High-field transport measurements revealed that collective depinning of localized charge carriers is essential to induce switching. The lack of collective charge motion is proposed as the origin of the switching suppression in the two-dimensional phase.

Resistivity noise in crystalline magnetic nanowires and its implications to domain formation and kinetics

We have investigated the time-dependent fluctuations in electrical resistance, or noise, in high quality crystalline magnetic nanowires within nanoporous templates. The noise increases exponentially with increasing temperature and magnetic field, and has been analyzed in terms of domain wall depinning within the Neel-Brown framework. The frequency-dependence of noise also indicates a crossover from nondiffusive kinetics to long-range diffusion at higher temperatures, as well as a strong collective depinning, which need to be considered when implementing these nanowires in magnetoelectronic devices.

Critical current suppression in [formula omitted] superconductors and its dependence on the defects concentration

For the first time, we have calculated the dependence of the critical current density j c on the defects concentration n d in a system simulating a HTSC layer. It was shown that j c ( n d ) dependence is non-monotonic and has a peak at the optimal concentration of defects n opt which is consistent with the known experimental data. We conclude that collective depinning—the reduction of effective pinning potential caused by increase of defects concentration—is the main mechanism for saturation and the subsequent decrease of j c .

Pinning-depinning of the contact line on nanorough surfaces

We study the pinning-depinning phenomenon of a contact line on a solid surface decorated by a random array of nanometric structures. For this purpose, we have investigated the contact angle hysteresis behaviour of six different wetting and non-wetting fluids with surface tensions varying from 25 to 72mN m(-1). For low values of the areal density of defects phi(d), the hysteresis H increases linearly with phi(d) indicating that "individual" defects pin the contact line. Then, from a given value of phi(d), the hysteresis H becomes to decrease with increasing phi(d), indicating a new kind of collective depinning. These two regimes were observed for all fluids used. In both cases, our experimental results are compared with the theoretical predictions for contact angle hysteresis induced by single or multiple topographical defects. We ascribe the decrease of H to the formation of cavities along the wetting front.

Depinning phase transitions in two-dimensional lattice Coulomb solids.

Collective depinning transitions in Coulomb solid-on-a-lattice systems are investigated in the low-charge-density limit. It is argued that depinning occurs at an inverted, continuous, dislocation unbinding transition of the Halperin-Nelson type. The depinning transition temperature is shown to be linear in the charge density, with a slope which is in good agreement with recent Monte Carlo simulations.